Launcher for extendible and retractable bridge structure

ABSTRACT

A bridge launcher which may be for example a stationary ground structure or a vehicle has attached thereto a bridge structure movable from a retracted position on the bridge launcher to an extended position at which it bridges a span. Both an end of the bridge structure and an end of a crossbeam are connected to a shaft on the launcher and an extendible element interconnects the end of an arm attached to the crossbeam with a point on the bridge structure close to the shaft. A further extendible element connected at one end to the launcher is operable to raise the bridge structure from its retracted position to a partially raised position at which the center of gravity is over the shaft. A further extension of the first said extendible element will then carry the center of gravity over a vertical plane through the shaft axis. At the fully extended position the crossbeam rests against an end wall of the launcher and the first said extendible member forms an adjustable brace for the extended bridge structure.

United States Patent 1191 Bouvet et a1.

1 1 Mar. 18, 1975 [221 Filed:

1 LAUNCHER FOR EXTENDIBLE AND RETRACTABLE BRIDGE STRUCTURE [75] Inventors: Claude Bouvet; Michel Eugene Terrien, both of Angers, France [73] Assignee: DPAGA-Bureau des Brevets & Inventions Republic of France, represented by the Minister of State for National Defense, Delegation Ministerielle Pour IArmement, Paris, France Aug. 31, 1973 211 Appl. No.: 393,610

[301 Foreign Application Priority Data FOREIGN PATENTS OR APPLICATIONS 1,550,111 11/1968 France 14/1 1.658.367 10/1970 Germany 1,926,859 12/1970 Germany 14/] [57] ABSTRACT A bridge launcher which may be for example a stationary ground structure or a vehicle has attached thereto a bridge structure movable from a retracted position on the bridge launcher to an extended position at which it bridges a span. Both an end of the bridge structure and an end of a crossbeam are connected to a shaft on the launcher and an extendible element interconnects the end of an arm attached to the crossbeam with a point on the bridge structure close to the shaft. A further extendible element connected at one end to the launcher is operable to raise the bridge structure from its retracted position to a partially raised position at which the center of gravity is over the shaft. A further extension of the first said extendible element will then carry the center of gravity over a vertical plane through the shaft axis. At the fully extended position the crossbeam rests against an end wall of the launcher and the first said extendible member forms an adjustable brace for the extended bridge structure.

17 Claims, 14 Drawing Figures LAUNCHER FOR EXTENDIBLE AND RETRACTABLE BRIDGE STRUCTURE BACKGROUND OF THE INVENTION This invention relates to an apparatus for extending a bridge structure articulated at one of its ends about a shaft of a bridge launcher, provided with a device for connection between the launcher and the bridge structure capable of swinging the bridge structure between a retracted, folded position in which it is retracted onto the launcher and an extended position in which it is deployed outside the bridge launcher to bridge a span.

Various bridge launchers are known which allow the positioning of bridges on obstacles or breaches so that they can be negotiated. However, the connecting de vices between these launchers and the bridge structure are complicated, heavy and cumbersome. They generally have numerous extendible elements such as hydraulic jacks whose dimensions are often considerable because of the weak lever arms that are used, especially in the case of floating bridges where it is necessary to provide emplacement at the ends of the launcher for propellers and their drive elements. The bulk occupied by these devices is considerable, and it is difficult to make light structures. Besides, when the bridge is in its extended operable position, the known connecting devices are not satisfactory for the transmission of stresses from the bridge to the launcher because they do not ensure perfect locking between these elements.

Thus, there exists a need for a new and improved bridge launcher-bridge structure connecting means which will overcome the difficulties of the prior art.

SUMMARY OF THE INVENTION the connecting device comprises a crossbeam articulated on the launcher shaft, a positioning mechanism comprising at least one extendible jack joining the end of the crossbeam opposite the shaft to the end of the bridge structure, forming a brace for the bridge structure in its operative extended position, and a drive mechanism comprising at least one extendible jack capable of effecting the swing of the crossbeam-bridge element assembly between a retracted folded position that corresponds to the transport position of the bridge structure and an operative extended position in which the crossbeam is bolted on the launcher.

By virture of this arrangement, no matter what the position of the bridge structure when it is extended, use of the crossbeam makes is possible to have a large lever arm with little bulk. which allows limitation of the number and size of the jacks. Moreover, it will be readily understood that use of the crossbeam allows considerable lowering of the point of action of the positioning jack so that by blocking the supply to the said jack, with the crossbeam bolted on the launcher, there is rigid locking of the bridge structure on the launcher, no matter what may be the inclination of the bridge structure. Also, stresses transmitted by the extendible jacks to the launcher in the course of crossing of the bridge by vehicles are reduced, so that the launcher structure may be lightened.

Advantageously the crossbeam has the form of a U articulated by its base on the launcher shaft, the mechanism for positioning comprising two extendible jacks joined to the ends of the arms of the said U. Thus, stresses undergone by the bridge element when it is in use are transmitted directly to the launcher and distributed uniformly on each of the arms of the crossbeam, which allows a perfectly rigid connection between the launcher and the bridge structure.

In a first embodiment, the drive mechanism comprises a jack connecting the crossbeam to the launcher. This drive mechanism makes it possible to cause the crossbeam-bridge structure assembly to swing until the jack reaches its dead center. Going past this dead center is then effected by shifting the center of gravity of the bridge structure beyond the perpendicular of the articulation shaft of the launcher by supplementary means.

In a second-embodiment, the drive mechanism comprises a deployment triangle articulated at one of its apexes on the launcher shaft, its other two apexes being joined by jack drive mechanisms to the launcher and the bridge structure respectively, the said jacks being adaptedto be actuated simultaneously in the same direction. This embodiment is more complex than the former one, but it allows avoidance of the problem of passing beyond the dead center.

BRIEF DESCRIPTION OF THE DRAWINGS There follows a detailed description of preferred embodiments of the invention which are to read together with the accompanying drawings wherein:

FIG. 1 shows a side view of the launcher with a connecting device joining it to a bridge structure according to a first embodiment of the invention, the bridge element being retracted onto the launcher.

FIG. 2 is a top view of the launcher of FIG. 1.

FIG. 3 is a front view of the launcher of FIG. 1.

FIGS. 4 to 7 show different stages in the operation of a bridge structure in two folded parts, the launcher having a connecting device as in the first embodiment.

FIG. 8 is a side view of the launcher with a connecting device joining it to a bridge element according to a second embodiment of the invention.

FIG. 9 is a top view of the launcher of FIG. 8.

FIG. 10 is a front view of the launcher of FIG. 8. FIGS. 11 to 14 show different stages in the operation of a bridge structure in two foled parts, the launcher having a connecting device as in the second embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, like numerals represent like elements throughout the several views.

Referring to the drawing, it will be seen a bridge launcher 1 supports a bridge structure 2 articulated at its end 3 about shaft 4 of the launcher. The launcher has a connecting device 5 that joins it to bridge structure 2.

This connecting device is constituted by a crossbeam 6 articulated on shaft 4 of the launcher, a positioning mechanism 7 comprising extendible hydraulic piston and cylinder jacks 8 joining the end of bridge structure 2 to the end of crossbeam 6 that is opposed to shaft 4;

a drive mechanism 9 capable of ensuring the swinging of the crossbeam-bridge structure assembly between a position corresponding to the transport position of the element and a position of use, and a bolting device fixed on the front wall of launcher 1.

The launcher which is of conventional design may be a stationary support structure such as a bridge piling or a vehicle, either a floating vehicle such as a ferry, an amphibious vehicle, a boat or a pneumatic float. The launcher is shown with wheels bolted in road position. It is apparent that its wheels can be retracted when the launcher is in aquatic use. This possibility is not illustrated, however. The launcher also has a propeller comprising a screw and the drive elements for the said screw. The propeller in the retracted position is indicated with dashed lines in FIGS. 1 and 8.

The bridge structure 2 is constituted by two parts 11 and 12 mutually articulated by a hinge that is not illustrated. When the bridge structure is folded onto the launcher, part 11 which is articulated on shaft 4 is retracted onto the launcher and covered by part 12. When the bridge structure is deployed outside the launcher, part 12 rests on the opposite edge of the span that is to be bridged.

The crossbeam 6 has the general configuration of a U whose base is articulated on shaft 4 of the launcher. The said crossbeam is constituted in fact by two bars that are parallel and articulated at one of their ends on the shaft and rigidly joined together by a transverse bar parallel to shaft 4, thus forming a kind of H shaped structure.

The positioning mechanism is constituted by two parallel, identical extendible hydraulic piston and cylinder jacks 8 These jacks, articulated at one of their ends on the ends of the crossbeam arms, are articulated at their other ends on part 11 of bridge structure 2.

Two variants of the drive mechanism 9 of connecting device 5 are illustrated and described. With reference to FIGS. 1 to 7, drive mechanism 9 is constituted by an extendible hydraulic piston and cylinderjack l3 joining crossbeam 6 to launcher 1. The said jack is articulated at one of its ends to the middle of the transverse bar joining the arms of the crossbeam, and at its other end on the launcher. The end of the jack disposed on the launcher is thus fixed, and only the end joined to the crossbeam is movable. When the crossbeam turns about shaft 4 of the launcher, jack 13 moves in a plane that is equidistant from and parallel to each of the planes described by the two jacks 8 of positioning mechanism 7.

With reference to FIGS. 8 to 14, the drive mechanism 9 is constituted by an intermediate member in the form of an isosceles deployment triange l4 articulated at its apex on shaft 4 of the launcher and at its two base corners to two jacks and 16. The opposite end of jack 15 is articulated on launcher 1 whereas that of jack 16 is articulated on part 11 of bridge structure 2. As noted above, triangle 14 is an isosceles triangle. When the bridge structure is folded on the launcher, the side of the triangle opposite the apex articulated about shaft 4 of the launcher is perpendicular to jacks l5 and 16. Triangle l4 and jacks l5 and 16 are in the same vertical plane. The end of jack 15 that is located on the launcher is fixed, and only its end joined to triangle 14 is movable. In contrast, the ends ofjack 16 connected on the one hand to part 11 of the bridge structure and on the other hand to triangle 14 are movable.

When thecrossbeam turns about shaft 4 of the launcher, the assembly constituted by triangle 14 and jacks l5 and 16 describe a plane that is equidistant from and parallel to each of the planes described by the two jacks 8 ofpositioning mechanism 7. FIG. 8 shows that jacks 15 and 16 are parallel to bridge element 2 when said element is in the transport position.

To examine the kinematic system used in deployment of the bridge structure in the situation in which the drive mechanism is a jack connecting the crossbeam to the launcher, reference is made to FIGS 1, 4, 5, 6 and 7. FIG. 1 shows that bridge structure 2 is in the transport position and that the parts constituting it are folded one on the other on the launcher. The rods of jacks 8 of the positioning mechanism and that of jack 13 of the drive mechanism are retracted. Crossbeam 6 is then applied against the bridge element. In FIG. 4, it is to be seen that the phase of deployment of the bridge element has begun, but that the element is still folded on the launcher. Jack 13 of the drive mechanism has caused crossbeam 6 to pivot slightly, by means of its rod whose elongation is controlled by a known device. During its pivoting, the crossbeam has brought along with it the rods of the jacks of the positioning mechanisms which are able to move freely. It is only when these elements have been sufficiently disengaged that jacks 8 are locked.

According to FIG. 5, the deployment of the bridge element has continued. As the action ofjack 13 is prolonged, the crossbeam has pivoted further about shaft 4 and the ends of its arms have continued to describe an arc that they started before, to assume the position of FIG. 5. The bridge element is no longer bedded on the launcher because the locked jacks of the positioning mechanism have carried it with them in their motion imposed by the ends of the crossbeam arms. In this figure it is to be observed that the bridge element has already described a quarter circle and that its center of gravity is on the vertical of shaft 4 of the launcher. Jack 13 is then at its maximal extension and reaches its dead 'point. With reference to FIG. 6, it is shown that the center of gravity of the bridge element has moved beyond shaft 4 after opening of its two parts. From the effect of the weight of this element, jack 13 has gone beyond its dead center while its rod is slightly retracted, forcing the crossbeam to continue its pivoting about the launcher shaft. Finally, according to FIG. 7, the bridge element is completely deployed outside the launcher, its part 12 resting with its end on the edge of the breach that is to be negotiated.

It is to be observed that jack 13 has completed its rotation in the course of which its rod is completely retracted, and that the crossbeam is locked in bolting device 10. To position the end of part 12 of the bridge element on the edge of the breach, the jacks of the positioning mechanism are actuated, said jacks constituting braces. Since the crossbeam is locked, the braces allow a broad sweep of positioning of the element, extending for example between +30 and --20% with reference to the horizontal plane as shown in FIG. 7. The hydraulic locking of these jacks then allowsimmobilizing of the bridge element in its position of use. Because of bolting device 10, the locking of the bridge element on the bridge launcher is realized. All the stresses applied on the bridge element in the course of its use are then transmitted to the launcher. It is obvious that the various operations described above, when executed in reverse directed, effect return of the bridge element to its original position as illustrated in FIG. 1.

To examine the kinematic system used in deployment of the bridge structure in the situation wherein the drive mechanism comprises a deployment triangle articulated by one of its apexes on the launcher shaft, and whose other two apexes are connected by jacks respectively to the launcher and the bridge structure, reference is made to FIGS. 8, ll, 12, 13 and 14.

In FIG. 8 the bridge structure is in the folded transport position. The rods of jacks 8 of the positioning mechanisms are retracted while those of jacks and 16 are extended. Crossbeam 6 is applied against the bridge structure while the two apexes of triangle 14, joined to jacks l5 and 16, are in the same vertical plane outside of the launcher, It may be noted in FIG. 11 that the stage of deployment of the bridge structure has begun but that it is still folded onto the launcher, Jacks 8 of the positioning mechanism, brought to maximum extension, have turned crossbeam 6 about shaft 4. During the pivoting of the crossbeam, jacks 15 and 16 and triangle 1.4 have retained the poisition that they had in FIG. 8. According to FIG. 12, the bridge structure has left its folded retracted position because the rods of jacks I5 and 16 have been retracted after having locked those of jacks 8 in the position of maximum extension. Since one of the ends of jack 15 is articulated on the launcher, the simultaneous retraction of the rods of jacks I5 and 16 forces the deployment-triangle to pivot downward. This pivoting consequently causes the upward pivoting of the bridge structure and allows continuation of that of the crossbeam. The center of gravity of the bridge element is now brought to the vertical.

Referring to FIG. 13, the center of gravity of the bridge element has now gone beyond the axis of shaft 4, but the problem of the dead center has not arisen. Retraction of the rods of jacks I5 and 16 continues while the deployment triangle and the crossbeam continue their revolution. The two parts of the bridge structure then begin to open under the effect of their weight and of auxiliary means.

FIG. 14 shows' the bridge structure completely deployed. The crossbeam is at the end of its rotation whereat it is joined fixedly to the launcher by means of bolting device 10. The rods of jacks l5 and 16 of the drive mechanism are returned within their cylinders while jacks 8 of the positioningmechanisms are adjusted to ensure contact of the end of part 12 with the edge of the span being bridged. Here again the positioning mechanism constitutes braces and allows a wide extent of positioning with reference to the horizontal plane. its locking and bolting of the crossbeam also constituting a rigid connection between the bridge structure and the launcher.

Of course, the various operations described above, when executed in reverse order, effect the return of the bridge structure to the original retracted positioni'ndicated in FIG. 8.

The deployment and folding of two parts of the bridge structure have not been described, since they are effected by means known per se.

Thus, because of the connecting device of the present invention, the lever arms of the jacks of the positioning mechanisms have a significant practically constant value, no matter what the position of the bridge structure. It is likewise to be observed that the stresses borne by the launcher are applied and distributed on the upper and lower parts of its front face, hence in zones that are readily reinforced, and that the stresses in the jacks and in the bridge structure are reduced. It is observed, moreover, that the mass of the connecting device is not great, and that it is possible to reduce still further those of the launcher and the bridge structure because of low stresses transmitted by the jacks when vehicles cross over the bridge.

Since the launcher is part of the roadbed and since the connecting device takes up little space, the openings in the bridge structure to allow passage of different organs of the connecting device are smaller, and the grating surface to be placed for operation of the bridge is negligible.

In the case of an amphibious launcher, the reduced bulk of the connecting device makes it possible to place a propeller in the center of the front wall. The crew then has a good output and allows ready manoeuverability of the launcher. When it is used on land, the propeller does not hamper visibility. To be protected, the propeller may even te totally retracted into the front face.

Although the invention has been describe in detail with respect to preferred embodiments thereof, it will be apparent that the invention is capable of numerous modifications and variations apparent to those skilled in the art without departing from the spirit and scope of the invention.

We claim:

I. In combination, a bridge launcher, a bridge structure articulated at one of its ends on a shaft-located on the bridge launcher for movement between a retracted position wherein the bridge structure rests on the bridge launcher and an operative extended position wherein the bridge structure is adapted to bridge a span, a crossbeam articulated on the said shaft and having at least one arm extending away from said shaft, a positioning means including a positioning extendible element interconnecting a first point on the said arm which is spaced away from the shaft to a point on the bridge structure near the end thereof which is articulated on the shaft, said positioning extendible element forming a supporting brace for the bridge structure in the extended position thereof, and a drive means including at least one drive extendible element for operatively interconnecting the bridge launcher and second point on the said arm between the shaft and the said first point thereon, whereby said drive means is operable, when extended, to move the crossbeam and hence also the bridge structure via said positioning extendible element from said retracted position at least to a partially extended position.

2. The invention of claim 1, said crossbeam comprising a pair of parallel arms, one end of each arm being articulated on the shaft, and having a cross bar extending parallel to the shaft and interconnecting the pair of arms, said positioning means including a separate extendible element for each of said arms.

3. The inventionof claim 2, said cross bar being spaced from and parallel to said shaft, said drive extendible element connected at one of its ends to said cross bar.

4. The invention of claim 1, said bridge launcher having a vertical end wall, said shaft axis being parallel to and located at the top of said end wall, said crossbeam arm being pivotable about said shaft axis from a position on top of the. bridge launcher in the retracted position to a position against and supported by the said wall in the extended position.

5. The invention of claim 4, said positioning extendible element being adjustable while the crossbeam is supported against and bolted with the said end wall for adjusting the inclination of the extended bridge structure.

6. The invention of claim 4, said bridge launcher being an amphibious vehicle, and including a propeller recessable into said end wall when not in use.

7. The invention of claim 1, wherein said drive means is operable to drive said bridge structure from the retracted position at least to a partially extended position at which the center of gravity of the bridge structure has at least reached a point above the said shaft.

8. In combination, a bridge launcher, a bridge structure articulated at one of its ends on a shaft located on the bridge launcher for movement between a retracted position wherein the bridge structure rests on the bridge launcher and an operative extended position wherein the bridge structure is adapted to bridge a span, a crossbeam articulated on the said shaft and having at least one arm extending away from said shaft, a positioning means including a positioning extendible element interconnecting a first point on the said arm which is spaced away from the shaft to a point on the bridge structure near the end thereof which is articulated on the shaft, said positioning extendible element forming a supporting brace for the bridge structure in the extended position thereof, and a drive means including at least one drive extendible element, said drive extendible element comprising an intermediate member pivotally connected to said shaft for movement in a plane perpendicular to said shaft, two other points on the intermediate member being pivotally connected to first and second extendible members, one of which is connected at its other end to the bridge launcher and the other of which is connected at its other end to the bridge structure near the shaft, the three said pivotal connections of the intermediate member forming an isosceles triangle with the pivot connection to the shaft being the apex thereof, whereby retracting both of said first and second extendible elements lifts the bridge structure off of the bridge launcher.

9. The invention of claim 8, said crossbeam comprising a pair of parallel arms, one end of each arm being articulated on the shaft, and having a cross bar extending parallel to the shaft and interconnecting the pair of arms, said positioning means including a separate extendible element for each of said arms.

10. The invention of claim 8, said bridge launcher having a vertical end wall, said shaft axis being parallel to and located at the top of said end wall, said crossbeam arm being pivotable about said shaft axis from a position on top of the bridge launcher in the retracted position to a position against and supported by the said wall in the extended position.

ll. The invention of claim 10, said positioning extendible element being adjustable while the crossbeam is supported against and bolted with the said end wall for adjusting the inclination of the extended bridge structure.

12. The invention of claim 8, wherein said drive means is operable to drive said bridge structure from the retracted position at least to a partially extended position at which the center of gravity of the bridge structure has at least reached a point above the said shaft.

13. In combination, a bridge launcher, a bridge structure articulated at one of its ends on a shaft located on the bridge launcher for movement between a retracted position wherein the bridge structure rests on the bridge launcher and an operative extended position wherein the bridge structure is adapted to bridge a span, a crossbeam articulated on the said shaft and having at least one arm extending away from said shaft, a positioning means including a positioning extendible element interconnecting a first point on the said arm which is spaced away from the shaft to a point on the bridge structure near the end thereof which is articulated on the shaft, said positioning extendible element forming a supporting brace for the bridge structure in the extended position thereof, and a drive means including at least one drive extendible element connected at one end to the bridge launcher, for raising both the bridge and the crossbeam from said retracted position to at least a partially extended position at which the center of gravity of the bridge and crossbeam are generally above the said shaft, and said positioning extendible element is in a position at which a further extension thereof will carry the said center of gravity onto the side of a vertical plane through the shaft axis away from the launcher and toward the extended position.

14. The invention of claim 13, said drive extendible element connected at its other end to a second point on the said arm between the shaft and the said first point thereon, whereby said drive means is operable, when extended, to move the crossbeam and hence also the bridge structure via said positioning extendible element from said retracted position to at least a partially extended position.

15. The invention of claim 13, said drive extendible element comprising an intermediate member pivotally connected to said shaft for movement in a plane perpendicular to said shaft, two other points on the intermediate member being pivotally connected to first and second extendible members, one of which is connected at its other end to the bridge launcher and the other of which is connected at its other end to the bridge structure near the shaft, the three said pivotal connections of the intermediate member forming an isosceles triangle with the pivot connection to the shaft being the apex thereof, whereby retracting both of said first and second extendible elements lifts the bridge structure off of the bridge launcher.

16. The invention of claim 13, said bridge launcher having a vertical end wall, said shaft axis being parallel to and located at the top of said end wall, said crossbeam arm being pivotable about said shaft axis from a position on top of the bridge launcher in the retracted position to a position against and supported by the said wall in the extended position.

17. The invention of claim 16, said positioning extendible element being adjustable while the crossbeam is supported against and bolted with the said end wall for adjusting the inclination of the extended bridge structure. 

1. In combination, a bridge launcher, a bridge structure articulated at one of its ends on a shaft located on the bridge launcher for movement between a retracted position wherein the bridge structure rests on the bridge launcher and an operative extended position wherein the bridge structure is adapted to bridge a span, a crossbeam articulated on the said shaft and having at least one arm extending away from said shaft, a positioning means including a positioning extendible element interconnecting a first point on the said arm which is spaced away from the shaft to a point on the bridge structure near the end thereof which is articulated on the shaft, said positioning extendible element forming a supporting brace for the bridge structure in the extended position thereof, and a drive means including at least one drive extendible element for operatively interconnecting the bridge launcher and second point on the said arm between the shaft and the said first point thereon, whereby said drive means is operable, when extended, to move the crossbeam and hence also the bridge structure via said positioning extendible element from said retracted position at least to a partially extended position.
 2. The invention of claim 1, said crossbeam comprising a pair of parallel arms, one end of each arm being articulated on the shaft, and having a cross bar extending parallel to the shaft and interconnecting the pair of arms, said positioning means including a separate extendible element for each of said arms.
 3. The invention of claim 2, said cross bar being spaced from and parallel to said shaft, said drive extendible element connected at one of its ends to said cross bar.
 4. The invention of claim 1, said bridge launcher having a vertical end wall, said shaft axis being parallel to and located at the top of said end wall, said crossbeam arm being pivotable about said shaft axis from a position on top of the bridge launcher in the retracted position to a position against and supported by the said wall in the extended position.
 5. The invention of claim 4, said positioning extendible element being adjustable while the crossbeam is supported against and bolted with the said end wall for adjusting the inclination of the extended bridge structure.
 6. The invention of claim 4, said bridge launcher being an amphibious vehicle, and including a propeller recessable into said end wall when not in use.
 7. The invention of claim 1, wherein said drive means is operable to drive said bridge structure from the retracted position at least to a partially extended position at which the center of gravity of the bridge structure has at least reached a point above the said shaft.
 8. In combination, a bridge launcher, a bridge structure articulated at one of its ends on a shaft located on the bridge launcher for movement between a retracted position wherein the bridge structure rests on the bridge launcher and an operative extended position wherein the bridge structure is adapted to bridge a span, a crossbeam articulated on the said shaft and having at least one arm extending away from said shaft, a positioning means including a positioning extendible element interconnecting a first point on the said arm which is spaced away from the shaft to a point on the bridge structure near the end thereof which is articulated on the shaft, said positioning extendible element forming a supporting brace for the bridgE structure in the extended position thereof, and a drive means including at least one drive extendible element, said drive extendible element comprising an intermediate member pivotally connected to said shaft for movement in a plane perpendicular to said shaft, two other points on the intermediate member being pivotally connected to first and second extendible members, one of which is connected at its other end to the bridge launcher and the other of which is connected at its other end to the bridge structure near the shaft, the three said pivotal connections of the intermediate member forming an isosceles triangle with the pivot connection to the shaft being the apex thereof, whereby retracting both of said first and second extendible elements lifts the bridge structure off of the bridge launcher.
 9. The invention of claim 8, said crossbeam comprising a pair of parallel arms, one end of each arm being articulated on the shaft, and having a cross bar extending parallel to the shaft and interconnecting the pair of arms, said positioning means including a separate extendible element for each of said arms.
 10. The invention of claim 8, said bridge launcher having a vertical end wall, said shaft axis being parallel to and located at the top of said end wall, said crossbeam arm being pivotable about said shaft axis from a position on top of the bridge launcher in the retracted position to a position against and supported by the said wall in the extended position.
 11. The invention of claim 10, said positioning extendible element being adjustable while the crossbeam is supported against and bolted with the said end wall for adjusting the inclination of the extended bridge structure.
 12. The invention of claim 8, wherein said drive means is operable to drive said bridge structure from the retracted position at least to a partially extended position at which the center of gravity of the bridge structure has at least reached a point above the said shaft.
 13. In combination, a bridge launcher, a bridge structure articulated at one of its ends on a shaft located on the bridge launcher for movement between a retracted position wherein the bridge structure rests on the bridge launcher and an operative extended position wherein the bridge structure is adapted to bridge a span, a crossbeam articulated on the said shaft and having at least one arm extending away from said shaft, a positioning means including a positioning extendible element interconnecting a first point on the said arm which is spaced away from the shaft to a point on the bridge structure near the end thereof which is articulated on the shaft, said positioning extendible element forming a supporting brace for the bridge structure in the extended position thereof, and a drive means including at least one drive extendible element connected at one end to the bridge launcher, for raising both the bridge and the crossbeam from said retracted position to at least a partially extended position at which the center of gravity of the bridge and crossbeam are generally above the said shaft, and said positioning extendible element is in a position at which a further extension thereof will carry the said center of gravity onto the side of a vertical plane through the shaft axis away from the launcher and toward the extended position.
 14. The invention of claim 13, said drive extendible element connected at its other end to a second point on the said arm between the shaft and the said first point thereon, whereby said drive means is operable, when extended, to move the crossbeam and hence also the bridge structure via said positioning extendible element from said retracted position to at least a partially extended position.
 15. The invention of claim 13, said drive extendible element comprising an intermediate member pivotally connected to said shaft for movement in a plane perpendicular to said shaft, two other points on the intermediate member being pivotally connected to first and second extendible members, one of whicH is connected at its other end to the bridge launcher and the other of which is connected at its other end to the bridge structure near the shaft, the three said pivotal connections of the intermediate member forming an isosceles triangle with the pivot connection to the shaft being the apex thereof, whereby retracting both of said first and second extendible elements lifts the bridge structure off of the bridge launcher.
 16. The invention of claim 13, said bridge launcher having a vertical end wall, said shaft axis being parallel to and located at the top of said end wall, said crossbeam arm being pivotable about said shaft axis from a position on top of the bridge launcher in the retracted position to a position against and supported by the said wall in the extended position.
 17. The invention of claim 16, said positioning extendible element being adjustable while the crossbeam is supported against and bolted with the said end wall for adjusting the inclination of the extended bridge structure. 